1. Technical Field
The present invention relates to pulse-width detection, and more particularly to pulse-width discrimination, that is, detection of pulses whose widths are at least as wide as a predetermined minimum pulse width, but within an allowable tolerance window.
2. Background Information
Pulse-width discrimination devices compare the width of an input pulse to a predetermined minimum pulse width, and output an indication signal only if the width of the input pulse is at least as wide as the minimum pulse width. The input pulse width must also fall within a predetermined tolerance window (permissible additional width variation from the minimum pulse width) before the indication signal is output. Thus, a pulse-width discriminator will output an indication signal only if the input pulse width is within the fundamental pulse duration (the minimum pulse width plus the tolerance window).
One method known in the art for generating the fundamental pulse duration is by using fixed delay lines to represent the minimum pulse width and the tolerance window. For example, "Studies of Logarithmic Radar Receiver Using Pulse-Length Discrimination" by Hansen, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-1, No. 3, Dec., 1965, pages 246-53, herein incorporated by reference, shows fixed delay lines comprising resistors, capacitors, and transistors. If the input pulse is within the tolerance window of the fundamental pulse duration, an indication signal is output. However, applications requiring adjustable pulse-width discrimination cannot use fixed delay lines, for they are difficult to adjust. Additionally, the use of resistors and capacitors in an RC network, although advantageous because of the high degree of accuracy attainable with precision RC components, are not advantageous for the discrimination of closely-spaced pulses. A major disadvantage exists due to the need to discharge the capacitor prior to discriminating subsequent pulses. Capacitor discharge time imposes a large dead time (the minimum interval, following a pulse, during which the circuit is not capable of repeating a specified function) which hinders the discrimination of pulses occuring during this dead time. Additionally, age, stress, and temperature deteriorate the RC network time interval accuracy.
Another method for generating minimum pulse widths is shown in an article entitled "Build a Pulse-Width Detector With a 555 Timer" by Sarpangal, EDN Magazine, Oct. 5, 1977, p. 93, herein incorporated by reference. Sarpangal configures an NE555 timer as a monostable multivibrator which is triggered by the leading edge of an input pulse. The timer generates a pulse width, representative of the minimum pulse width, by an RC network. If the input pulse width is wider than the minimum pulse width, the circuit outputs a pulse corresponding to the input pulse width less the minimum pulse width. Sarpangal thus is not concerned with a tolerance window, but rather the excess pulse duration following the minimum pulse width. Additionally, the use of the RC network to generate the minimum pulse width is not advantageous for the reasons stated above.
To overcome problems associated with both fixed delay line and RC networks, other systems have been developed. For example, U.S. Pat. No. 3,949,199 issued to Odom, herein incorporated by reference, describes a pulse-width decoder utilizing digital components to both set the minimum pulse width and tolerance windows, and reset the decoder. The minimum pulse width is set by a first counter network which counts pulses generated by an oscillator, the oscillator being enabled by the input pulse. If the input pulse is longer than the minimum pulse width, subsequent oscillator pulses are counted by a second counter network which outputs a pulse to a multi-stage shift register every N oscillator pulses. N determines the tolerance window, and the shift register output determines the harmonic of the tolerance window. For example, if the output is at the second stage of the shift register, the input pulse width is at least the minimum pulse width, and between 2N and 3N in width. Thus, the input pulse width can be resolved to an N tolerance window, provided that the input pulse width does not exceed N times the number of stages of the shift register. To expand the maximum pulse width receivable, Odom includes a feedback network between the shift register outputs and the second counter network which increases the value of N as the output stage increases. The trailing edge of the input pulse disenables the oscillator and causes additional circuitry to reset the counters and shift register. Accordingly, Odom detects pulses having at least a minimum width, and indicates the width of the input pulse according to the harmonic of the tolerance window.
Although Odom overcomes the problems associated with the dead time inherent in RC networks, Odom cannot accurately determine the width of a pulse except within the range of N multiplied by the oscillator frequency. Also, the width of any detectable pulse must fall within a range of N multiplied by the number of stages of the shift register. Accordingly, if N is large, the pulse-width accuracy is poor; if N is small, the maximum pulse-width detectable is small. Additionally, although the window range (N) is variable, changing one window range automatically changes the pulse-width detection range for all subsequent tolerance windows, for the window ranges are contiguous. Furthermore, fundamental pulse durations and harmonies thereof are not detected. Rather, tolerance window harmonics, after an initial minimum pulse width is satisfied, are detected.